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This paper established a necessary condition for the sintering of powder compacts by examining the total free energy balance in terms of the particle size, neck size and contact number. The thermodynamic analysis of the proposed model clarifies the relation of shrinkage (q) of powder compact-contact angle ()-relative density at a given dihedral angle (e) of a grain boundary. Faster densification proceeds in the region with a larger coordination number (n) of particles at a small q value. A large shrinkage is needed to eliminate the large pores formed in the structure of small n value. Full density can be achieved in the range of 117° < e < c, where c is the critical dihedral angle allowing the shrinkage required for full densification. The derived concepts are effective to interpret the densification of hierarchical particle clusters. The relative density of ceria powder compact approached nonlinearly unity with decreasing ratio of pore size (r(P)) to grain size (r) and this tendency was well expressed by the present densification model. The influence of grain growth on the densification of powder compact and size of large pore isolated in a dense matrix are also quantitatively discussed. 相似文献
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Rotational molding involves powder mixing, heating and melting of powder particles to form a homogeneous polymer melt, as well as cooling and solidification. The densification of a loose powder compact into a homogeneous melt occurs over a wide range of conditions as the material passes from a solid state into a melt state. The numerical simulation of the non‐isothermal melt densification in the rotational molding process is presented in this work. The simulation combines heat transfer, polymer sintering and bubble dissolution models, and is based on an idealized packing arrangement of powder particles. The predictions are in general agreement with experimental observations presented in the literature for the rotational molding of polyethylenes. The simulation allows for systematic and quantitative studies on the effect of molding conditions and material properties on the molding cycle and molded part density. Results indicate that the densification process is primarily affected by the powder characteristics, which are accounted for in terms of the particle size and the particle packing arrangement. The material rheological properties become increasingly important as the powder characteristics lessen in quality. The simulation demonstrated that while certain combinations of processing conditions help reduce the molding cycle, they have a detrimental effect on the densification process. 相似文献
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J. Greener 《Polymer Engineering and Science》1986,26(8):534-542
The effect of pressure on the densification of amorphous polymers in the Injection-molding process is examined. Density distributions in molded polystyrene slabs were measured for several well-defined molding histories. In all cases the density of the molded part was spatially inhomogeneous, and its distribution in the slab was closely related to the pressure and temperature histories that prevailed in the molding cavity during the process cycle. The density profile in the gapwise direction followed a characteristic “parabolic” pattern with a minimum at the midplane of the slab. A simple phenomenological model, based on the pressure-induced densification effect, was constructed to explain the observed density profiles, and close agreement with experimental data was found. Annealing of the molded article at a high temperature (<Tg) caused the density to decrease overall and become more uniform across the part. This is generally consistent with volume-recovery data for the densified material, which were generated independently in a controlled pressure-densification experiment. 相似文献
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The nature of powder flow and its effect on particle deposition in rotationally molded parts were considered in this work. Experiments were carried out to observe the effects of various parameters, such as particle characteristics and operating conditions, on the deposition patterns of polyethylene powders and micropellets. The results indicate that the polymeric powders were cohesive enough to prevent size segregation at ambient temperature; however, segregation occurred when particles that had a smooth surface and regular shape were used. During processing, however, a new phenomenon of reverse cohesive segregation was observed. The results showed that the final deposition patterns are controlled primarily by the initial segregation patterns, as well as by the heating rate and rotation speed, which affect the evolution of adhesive forces between particles during heating and melt deposition process. An order‐of‐magnitude analysis was conducted to evaluate the development of cohesive forces between particles, and to estimate their effects on the movement of particles. This study provides a better understanding of the flow characteristics of polymer particles during the rotational molding process, which is very important in the development of techniques for fabricating composites and multilayered products. Polym. Eng. Sci. 45:62–73, 2005. © 2004 Society of Plastics Engineers. 相似文献
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Motion control parameters of rotational molding can affect process efficiency and product quality. Different motion control schemes will lead to varied powder flow regimes exhibiting different levels of mixing and temperature uniformity. The change in nature of powder flow during a molding cycle suggests that varying the rotational speed could improve the powder mixing and temperature uniformity, therefore potentially reducing processing time and energy consumption. Experiments completed investigating powder flow under uniaxial rotation show that savings of up to 2.5% of the heating cycle time can be achieved. This validates the hypothesis that altering the rotational speed to maintain the ideal powder flow throughout the heating cycle can be utilized to reduce the time taken for all the polymer powder to adhere to the mold wall. The effect of rotational speed on wall thickness uniformity and impact strength were investigated and discussed. Results show a strong influence of rotational speed (and powder flow) on the wall thickness uniformity of the moldings with wall thickness uniformity deviations of up to 50% found (within the 2–35 RPM speed range tested). 相似文献
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George Gogos 《Polymer Engineering and Science》2004,44(2):388-394
Closed form solutions have been obtained for bubble dissolution in typical polymer melts encountered in rotational molding. The solutions are in excellent agreement with experimental data available in the literature. Using these solutions, it is shown that under typical rotational molding conditions the polymer melts may be almost saturated. As a result, bubble shrinkage occurs over long periods. Depending on the degree of saturation, surface tension may contribute substantially to the concentration gradient that drives bubble shrinkage. It is also shown that a pressure increase imposed on a nearly saturated polymer melt leads to a steep concentration gradient at the bubble/melt interface that can cause extremely fast bubble shrinkage. Applied to the rotational molding process, such a pressure increase can result in substantial cycle‐time shortening through elimination (or reduction) of the currently used excessive heating. A further benefit may be that additional resins, which at present cannot be used because of oxidation at sustained high‐temperatures, can become available to the rotational molding industry. Under the under‐saturated conditions created by a pressure increase, the effect of surface tension on the rate of bubble shrinkage is negligible. 相似文献
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We present a detailed theoretical model of the rotational molding process, and identify the key dimensionaless groups affecting the process cycle time. This theoretical model is employed to create differential and lumped parameter numerical models, as well as a simple closed form estimate for the time required for complete powder deposition. Both numerical models give results that are in very good agreement with experimental data available in the literature. The closed form solution gives good predictions over a wide range of processing parameters. In addition, the effects of variations in the dimensionaless groups on processing time are evaluated. 相似文献
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综述了滚塑成型发展历史、工业概况、材料、应用、市场及其动向,重点讨论了滚塑成型对材料(树脂和配混料)的要求和优缺点,比较了滚塑成型和其他中空成型方法的材料、成型、设计、模具和最终制品性能。 相似文献
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Bharat Indu Chaudhary Elizabeth Takcs John Vlachopoulos 《Polymer Engineering and Science》2001,41(10):1731-1742
Rotational molding is a zero shear process used to manufacture hollow plastic parts. One disadvantage of this process is long cycle times, which are significantly affected by the sintering rates of thermoplastic powder. The objective of this work was to evaluate low molecular weight additives as sintering enhancers for polyethylene and to validate the results in rotational molding. The following additives were blended with linear low‐density polyethylene: mineral oil, glycerol monostearate and pentaerythritol monooleate. The additives resulted in decreased melt viscosity and/or elasticity at low shear rate. The reduction in melt elasticity was particularly significant. Sintering studies confirmed that the additives resulted in significantly faster coalescence. In uniaxial rotational molding, the decreased melt viscosity and elasticity obtained with mineral oil were observed to result in much faster densification and bubble removal. Part thickness was uniform and there was no warpage. Adding mineral oil to polyethylene reduced the cycle time in uniaxial rotational molding and the peak impact strength was identical to that obtained without any additive. Biaxial rotational molding experiments confirmed that the use of mineral oil resulted in shorter cycle time without sacrificing peak impact strength. 相似文献
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E. Mendizabal J. M. Candia V. Gonzalez-Alvarez C. F. Jasso-Gastinel L. Cruz 《乙烯基与添加剂工艺杂志》1994,16(2):91-95
In this work we present the optimization of the processing conditions for PVC plastisol rotational molding. The effects of oven temperature, processing time, rotational, speed and amount of plastisol over the degree of curing, physical appearance, and mechanical properties of the molded articles were studied. Also, using a simple mathematical model to simulate the temperature profiles of the plastisol inside of the spherical mold as a function of time, the viscosity change of the plastisol with time is reported. A rotational molding machine laboratory size was used for the experiments. The oven (at different points) as well as the mold (at the inside and outside of the cavity) temperatures were measured as a function of time in order to get a better understanding of the curing process. Such data in conjunction with the model gives the support for process optimization and control. 相似文献
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Rotational molding is a process for manufacturing hollow or open‐sided plastic products using a rotating mold subjected to heating and then cooling. The process is attractive for the production of stress‐free objects at a competitive cost. In this article, a modified model for heat transfer in rotational molding is proposed, which assumes that the heat transfer at the mold‐powder interface is because of convection, whereas the powder particles are heated up by conduction. Heat transfer through the mold–air contact is also included. A source‐based formulation is used for modeling the layer‐by‐layer nonisothermal deposition of plastic. The reduced heat transfer due to warpage is calculated by using a modified heat transfer coefficient. Good overall agreement is found between the cycle times as predicted by the model and the experimental data. The model is then used for calculating the cycle time for particulate composites, based on their effective properties. A reduction in the cycle time is observed in the case of reinforced composites. This is attributed to the increase in thermal conductivity of the particulate composites and the reduced mass fraction of the polymer. Numerical calculations of the cycle time for the glass‐bead reinforced composites are found to be in good agreement with the experimental results. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers 相似文献
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A mathematical model is established to describe the thermal debinding process of polymeric binder from a powder injection molding compact. The model takes into account of the thermal degradation of liquid polymer into liquid volatile fragment, the evaporation of liquid volatile fragment, the capillary driven liquid phase transport, the binary diffusion in solution, the convection and diffusion of gas phases, and the heat transfer in a porous medium. The proposed model is solved numerically based on a finite volume method and validated with experimental data. Based on the numerical results, the binder removal, the pressure buildup, the binder distribution, the mass transfers, and the removal mechanisms during thermal debinding are studied. 相似文献
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In this work, the rheokinetic of polyurethane crosslinking was studied by different methods: differential scanning calorimetric (DSC), rheometry, and infrared spectrometry. The conversion ratio and the glass transition temperature were followed by time of reaction. The results of the isothermal and nonisothermal test were compared. The evolution of viscosity was measured at different frequencies. The intersection of these curves is considered as gel point. A simplified mechanism has been proposed for crosslinking reactions. Based on this mechanism, a kinetic model describing the evolution of reactive system was developed. This model then was compared with the results of experiments performed by infrared spectrometry. The time-temperature-transformation diagram was established showing the evolution of physical state change of the reactive system. This diagram may be used to evaluate the zone of rotomoldability of the reactive polyurethane. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012 相似文献
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Paola Stagnaro Roberto Utzeri Adriano Vignali Giulio Falcone Salvatore Iannace Fabio Bertini 《应用聚合物科学杂志》2021,138(5):49766
Innovative composites based on polyethylene (PE) filled with hollow glass microspheres (HGMs) were formulated and successfully prepared as suitable plastic materials for rotational molding technology. The HGMs here used allow to attain lightweight materials with a reduced resin content and appealing aesthetical qualities. To enhance filler dispersion and phase adhesion, thus improving the ultimate properties of the composite materials, two compatibilization strategies were adopted: namely, surface modification of HGMs by dodecyl(triethoxy)silane or addition during mixing of a maleinized PE as in-situ coupling agent. The effectiveness of the surface treatments on HGMs was assessed by attenuated total reflectance Fourier-transform infrared spectroscopy and thermogravimetric analysis investigations. PE-based composites at various HGMs contents (5, 10, and 20 wt%) were prepared by melt blending. Morphology of untreated and modified HGMs, their dispersion in the composites as well as filler/matrix adhesion were investigated by SEM microscopy. Thermal, rheological and mechanical properties of the composites were studied in comparison with neat PE. Rotational molding tests carried out both in laboratory and industrial site demonstrated the feasibility of producing lightweight plastic items (weight reduction up to 17%) of excellent aesthetics on a large scale. 相似文献